The present invention relates to sensors for detecting acceleration and angular velocity and, in particular, to a sensor for detecting acceleration and angular velocity based on a force applied to a weight body.
Industrial machinery and electronic devices are often equipped with units for detecting physical quantity such as acceleration and/or angular velocity. For this reason, many types of small-sized and high-accuracy acceleration sensors and angular velocity sensors have been developed. In particular, demand for multiaxial acceleration sensors that can detect two- or three-dimensional acceleration and multiaxial angular velocity sensors that can detect bi- or tri-axial angular velocity has been increasing. For example, recently, many digital cameras have a multiaxial acceleration sensor and/or a multiaxial angular velocity sensor incorporated therein for stabilization control. For this type of application, it is an important issue to reduce the size of sensors, and it is requested that one small-sized sensor can detect both acceleration and angular velocity (such a sensor is commonly called a motion sensor).
In response to such a request, the inventor has proposed sensors capable of detecting acceleration and angular velocity. For example, Japanese Unexamined Patent Publications No. 8-068636A and 2002-350138A disclose sensors for detecting acceleration and angular velocity by electrically detecting the displacement of a weight body (oscillator) due to acceleration or angular velocity as deflection of a piezoelectric element. Japanese Unexamined Patent Publication No. 2005-031096A also discloses a sensor for detecting acceleration and angular velocity by electrically detecting the displacement of a weight body (oscillator) as a change in the capacitance of a capacitor.
Such a type of sensor including a piezoelectric element utilizes the properties of piezoelectric elements that mechanical deformation causes charge generation. Therefore, “dynamic displacement of the weight body (transitional motion)” can be detected, but “static displacement of the weight body (deviation from a fixed position)” cannot be detected. Consequently, as for angular velocity detection based on the motion of the weight body, both static angular velocity (angular velocity of rotational motion at a constant speed in a constant direction) and dynamic angular velocity (angular velocity of rotational motion at a time-varying speed and/or in a time-varying direction) can be detected. However, as for acceleration detection, dynamic acceleration (acceleration of a time-varying amount in a time-varying direction) can be detected, but static acceleration (constant acceleration due to, for example, gravity) cannot be detected in this type of sensor. For example, applying a force to the piezoelectric element causes a predetermined amount of charge to be generated initially as a transient response. However, if the applied force is constant, no charge is to be generated after the stabilization of the measurement system. Therefore, constant acceleration due to, for example, gravity (static acceleration) cannot be detected.
On the other hand, such a type of sensor including a capacitor, which can detect the displacement itself of the weight body as a capacitance (distance between the electrodes) of the capacitor, can detect static and dynamic angular velocity as well as static and dynamic acceleration. However, since it is necessary to install wirings for each of the pair of electrodes that constitute the capacitor, the entire structure of the sensor inevitably becomes complicated. Particularly, in the case of an arrangement in which air that serves as a damper is removed from around the weight body such that the interior is evacuated in order to obtain detection values with high accuracy, it is necessary to install wirings for the electrodes inside while keeping the vacuum state, which requires a very complicated structure.